This invention relates generally to a light emitting device and more particularly is concerned with monitoring and controlling the operation of a light emitting device such as a globe.
Light emitting devices such as incandescent globes, intended for example mainly for the domestic market, have been fabricated with two or more elements. If one element should fail then the globe is rotated so that additional contacts on the globe are connected to an electrical supply, thereby allowing a second element to be energized in place of the failed first element. Globes for vehicles with two filaments (dim/bright) are also known in the art U.S. Pat. No. 4,580,079 to Koo allows for a surge of current to blow a fuse that will result in a mechanical switchover to occur.
Failure of a light source, e.g. a globe, in a lighting installation such as a building is normally easily detectable and non-critical for appropriate action may readily be taken. On the other hand for example, if a globe fails in a vehicle or in a difficult to reach installation, such as on a mast, then failure of the light source may not always be easily detected or ascertainable and, even if detected, appropriate rectifying action may be difficult to take without the use of workshop facilities or special tools.
Many vehicles do have circuits which monitor the operation of electrical components such as headlights, brake lights, tail lights and so on. If a headlight should fail this will possibly be noticed by the driver of the vehicle but it may be difficult for the driver to take immediate remedial action to replace the globe. On the other hand the driver of a vehicle would be quite oblivious of the failure of a tail or brake light unless the vehicle includes light monitoring circuits with appropriate indicators.
It would be convenient if preventative actions could be taken during regular scheduled maintenance. For example, when a vehicle is serviced, it would be convenient if all light bulbs due to fail shortly could be replaced. It would also improve road safety.
The invention provides a light emitting device which includes at least a first light source which is connectable to an energy source, a second light source, detector means for detecting a failure of the first light source, and switch means, responsive to the detector means, for enabling the connection of the second light source to the energy source when the detector means detects failure of the first light source.
The light emitting device may include delay means, to which the switch means is responsive, for delaying the said connection of the second light source to the energy source for a predetermined time interval. This allows the failure of the first light source to be detected by external failure detection means, for example in a light monitoring system of a vehicle.
The light emitting device may include indicating means for providing an indication of the failure of the said first light source. Such indicating means may take on any suitable form and, for example, may consist of a visual indicator.
The indicator means may alternatively or additionally cause a recognizable variation in operation of the second light source. For example the indicator means may cause the second light source to flicker for a predetermined time interval or in a predetermined flashing sequence.
The device may include a control circuit which preferably is in the form of an integrated circuit, for controlling the operation of at least the switch means. The detector means is preferably formed integrally with the control circuit,
The light sources in the light emitting device may take on any suitable form and for example may comprise incandescent elements or filaments, halogen quartz units, discharge devices, light emitting diodes (LED""s) and the like. The scope of the invention is not limited in any way in this regard.
In a preferred embodiment of the invention the light emitting device is packaged or configured in the shape of a conventional light source, such as a bulb or globe, so that the light emitting device of the invention can be used as a direct replacement of the conventional light source.
It therefore falls within the scope of the invention for the first and second light sources to be located inside an enclosure which is at least partly translucent and for the said detector means and switch means to be mounted to or located within the enclosure.
In another embodiment at least the second light source, i.e. the back-up or spare light source, and the failure detector means, are combined in the form of a conventional light bulb which can be used in conjunction with the first light source, i.e. as ordinary (primary) light bulb, and which will be energized only if the primary light bulb fails.
Various parameters which are dependent on the correct functioning of the first light source can be monitored to detect failure of the first light source. For example the heat which is generated by the first light source can be monitored. The current which is drawn by, or the voltage drop over, the first light source can also be measured. Closely linked to the foregoing is a direct or indirect measurement of the resistance of the first light source.
Another possibility is to monitor the intensity of the light, at visible or infrared frequency, emitted by the first light source. One could also measure the strength of a magnetic field associated with the first light source, e.g. the magnetic field generated by current flowing through the first light source. The invention is not limited in this regard.
In a preferred embodiment the device includes an integrated circuit and at least the detector means and the switch means are incorporated in the integrated circuit. Any other suitable control or switching functions may be integrated in, or be provided by, the integrated circuit. The detector means may for example be a light sensor, such as a light sensitive device or any equivalent device, which is fabricated on the integrated circuit e.g. during a standard CMOS production process. The switch means, which may be responsive to controlling logic, can also be included in the integrated circuit.
The integrated circuit may be designed to work intermittently so that its operating life is prolonged. For example the integrated circuit may be designed to work only for a short period after it is powered up. This is specifically to prevent fast degradation of the integrated circuit due to the effects of heat.
The integrated circuit can also be used to control the duty cycle of any of the light sources, and particularly the back-up light source, so that such light source emits light of a desired intensity.
If power can be provided to the integrated circuit permanently, irrespective of either light source being energized, then the integrated circuit can be provided with a memory function and can indicate a failure of the first light source even if power is not being applied to the first light source nor to the second light source. This can for example be in a period immediately after power to each light source is turned off.
In a specific embodiment the detector means may include a light sensor and the output of the light sensor, in response to incident light, can be measured in volts or in any other suitable way. When the power to the first light source is applied the integrated circuit can store the output of the light sensor using a sample and hold technique. The spare or second light source can then be activated. After a short period to let the second light source warm up and produce light another measurement can be taken and the new measurement can then be compared with the value which was stored. If the difference between the two values is greater than a defined threshold the light source will be regarded as having failed. The process can be repeated a few times to ensure that the measurement is accurate.
The light sensor and, optionally, the integrated circuit can be located at positions at which the effects of sunlight and also of heat generated by the light sources are relatively low.
The number of light sources to monitor for failure can be more than one and the number of spare or second light sources can also be more than one.
Although the invention is described particularly with reference to 12 volt automotive applications the invention is not restricted in this regard for many other applications are possible including, but not limited to, AC and DC lighting in buildings of all kinds, whether business, industrial or domestic, and in other installations.